This invention relates to information transmission systems in which information is transmitted between two or more stations in digital form. More particularly, this invention relates to the reduction of errors in digital information transmission systems.
In the field of information transmission, it is common practice to convert information from analog to digital form prior to transmission from a station at a first location and to reconvert the information received at a second location from digital to analog form. In a typical system, the analog-to-digital conversion is accomplished by sampling successive portions of the analog input signal at a rate sufficient to permit conversion in a theoretically error-free manner under idealized conditions and generating a substantially constant level signal for the duration of each sampling period, the magnitude of the constant level signal during any given period being representative of the magnitude of the analog signal at the instant of sampling. The magnitude of the constant level signal is limited to a relatively small fixed number of possible values over the entire predetermined amplitude range of the analog input signal, a process termed quantizing, and each value is assigned a different amplitude range or quantizing interval so that all signal amplitudes lying within a specific quantizing interval are converted to a constant level signal having the same magnitude. For example, in a 7 bit binary system an analog input signal having amplitudes lying in the range from 0 to 1.28 volts may be quantized into different levels each having a range of 0.01 volts so that input signals having amplitudes lying in the 0 level range from -0.005 to 0.005 volts are converted to a 0 volt level signal; input signals having amplitudes lying in the range from 0.005 to 0.015 volts are converted to a constant level signal having a magnitude of 0.01 volts; signals from 0.015 to 0.025 volts are converted to a constant level signal having a magnitude of 0.02 volts; etc. The voltage magnitudes 0.005, 0.015, 0.025 defining the end points of each range are termed the transition points.
At the receiving station, the information transmitted in digital form is ordinarily reconverted to analog form which is accomplished in the inverse manner from that described above. Such systems have found wide application, and are increasingly being used in telephone systems for transmitting speech or other analog information.
Such systems are typically designed to operate over a predetermined range of analog input signal frequencies. For example, in a telephone system application, this range is ordinarily in the audible range from about 300 to about 3,400 HZ. System response is limited to this range by filtering the analog input signals prior to the analog-to-digital conversion by means of a band pass filter having a pass band characteristic lying in the 300-3,400 HZ range; and by filtering the reconverted analog signal with a post sampling filter having a substantially identical pass band characteristic.
Such systems suffer from the disadvantage of being susceptible to random disturbing signals upstream of the analog-to-digital converter, or ADC, and lying in the frequency response range of the system, which signals are conveniently termed noise signals, as opposed to information signals whose information content is to be transmitted to the receiving station. In the presence of noise signals, the information content desired to be transmitted and received can be masked and erroneously manifested at the receiving end of the system. Ideally, under idle channel conditions, i.e. when no information is present on the input side of the system, the output of the ADC should have a constant zero level value. In practice, however, in a typical ADC the zero level range drifts. Thus, a random or spurious disturbing signal having even an extremely small amplitude can cause the ADC to generate an output signal one quantizing the value higher or lower than zero, if the zero level value has drifted close to a transition point. This erroneous output signal is then reproduced as an erroneous analog signal at the downstream digital-to-analog converter, or DAC.
In systems using a multi-channel input which is sequentially coupled to the ADC, i.e. a multiplexed multi-channel system, noise in the form of cross talk from a nearby channel is typically present. Since the cross talk noise signal has the spectral content of speech and thus lies within the frequency response range of the system, cross talk signals of even extremely small amplitude can pass through the system band pass filter and alter the magnitude of the sampled analog information input signal to a value lying within the next quantizing interval, particularly when the input signal alone is very close to a transition point. As a result, the ADC generates an erroneous output signal which is reconverted to analog form by the DAC. Since the spectrum of this signal is fundamentally a speech spectrum, any such noise can not be filtered out by the post sampling downstream from the DAC.
A third type of noise, termed "quantizing error", arises from the inability of the ADC to recognize amplitude changes in the analog information input signal which lie within a quantizing level. Such changes are not converted by the ADC and thus will not be reproduced in the DAC even though the actual magnitude of the analog information input signal may have changed between successive samples.
Attempts have been made to design systems of the above type with reduced sensitivity to idle channel, cross talk and quantizing error noise. In some systems, the number of quantizing intervals used to represent the input signal has been increased, thus decreasing the size of each quantizing interval. For systems using binary encoding, it can be shown that adding n bits or 2.sup.n quantizing intervals reduces the effect of this noise by 6n db in an ideal case, provided that the analog noise in the system remains small compared to the size of the quantizing interval. Another technique is to introduce a circuit which has greater gain for small signals than for large amplitude analog signals, termed a compressor, upstream of the ADC, and a circuit having the inverse gain characteristics of the compressor, termed an expander, downstream of the DAC. The compressor-expander arrangement effectively reduces the size of the quantizing intervals for small amplitude signals and correspondingly reduces the adverse effects of idle channel and cross talk noise. However, this arrangement has the disadvantage of introducing a non-linear response over the entire amplitude range of the analog input signals and requires further corrective circuitry in order to avoid increase in quantizing error noise.
All of the above efforts to reduce the adverse effect of idle channel cross talk and quantizing error noise on a signaling system have been found to be somewhat effective, but suffer from the serious disadvantage of increasing the cost of the circuitry required for an operable system. In highly sophisticated systems, such as those used in the telephone signaling art, this increased cost is greatly multiplied by the total number of circuits employed.